Compensation of losses and defects in telecine devices

ABSTRACT

A signal produced by a telecine suffers from losses and defects caused by differences in response to incident light between different areas of the screen area scanned. These differences may arise from burning of the screen, blemishes, dirt in the system, differences in grain size of phosphor particles, missing particles, and losses in the internal optical system of the machine. The invention divides the scanning area into a correction map and devises for each area a correction factor based on the response of that area to incident illumination. When a defect is detected, video data from an adjacent area is substituted. Correction factors are held in a look-up RAM and output to a multiplier where they are multiplied with video data. The video data input to the multiplier may be suppressed and a test pattern may be loaded into the multiplier. Burn at the ends of video lines may be reduced by reducing the intensity and/or dwell time of the scanning spot on any one area of the screen in the vicinity of the line end.

FIELD OF THE INVENTION

This invention relates to telecine machines and in particular to thecorrection of shading burn, blemish and dirt errors. The invention isalso applicable to film writers; that is, telecine machines which areoperated to write from an input video signal.

BACKGROUND AA

Attempts have been made to minimise errors caused by deficiencies in theCathode Ray Tube (CRT) or Photo-Multiplier Tube (PMT). Shadingcorrection has been used on TV cameras and telecine for many years andhas generally taken the form of a set of waveforms which are predefinedor adjusted to be the reciprocal of the shading error, these waveformsbeing multiplied by (in the case of linear signals), or added to (forlogarithmic signals) the uncorrected video signals to produce signalswhich are substantially free from shading errors.

This method of correction is satisfactory for smooth and symmetricalerrors such as may be caused by non-uniform lens transmissioncharacteristics. However, errors caused by variations in efficiency ofthe phosphor layer in CRTs are not consistent and require thecombination of adjustable proportions of many different waveforms toachieve a satisfactory correction. The same considerations are true forvariations in cathode sensitivity of PMTs and graduations of colourfilters. Consequently, to obtain satisfactory correction, a large numberof controls requiring a complex alignment is necessary. Even then it isnot possible to correct isolated patches on the picture.

Another type of error is known as burn. Burn errors are those caused bythe scan spending a longer time at some locations on the CRT face thanat others. The result is localised solarisation of the glass or stainingof the phosphor which results in distinct steps in the light output atthose locations. Burn errors cannot be corrected by the type of shadingcorrector described above since they generally display a series of sharpedges. It has been proposed to use a separate burn corrector which usesan additional PMT which looks directly at the CRT face to measure theburn errors, and then calculates the reciprocal of the error waveform,before multiplying it together with the video waveforms to producecorrected video waveforms.

This proposed method has not proved wholly satisfactory as it has thedisadvantage that non-uniformities in the burn correction optical pathor due to burn PMT sensitivity variations (which may be verysignificant) will be applied to the video signal, resulting in a moredifficult task for the shading corrector.

A further source of errors are blemishes and dirt. Blemishes are smallsharply focused spots of no light output from the CRT which are causedby missing particles of phosphor or debris on the phosphor surface. Dirton the CRT faceplate will also appear as dark spots.

Neither of these errors can be corrected by the shading correctoroutlined above. However, one or other of the dirt or blemish errors canbe corrected using the burn corrector described. Both errors cannot becorrected at once as the burn PMT is off the optical axis and thus givesparallax errors so that when adjusted to correct for blemishes itproduces a correction for dirt at the wrong picture location. Fittingthe burn PMT on the optical axis is undesirable as a mirror system wouldhave to be used involving a consequent loss of light and deteriorationin the telecine signal to noise ratio.

Another source of errors is the phosphor grains themselves. The granularstructure of the CRT phosphor results in random variations in the lightoutput which are of small size and amplitude. These errors cannot becorrected by the shading corrector described above but can be improvedby the burn corrector when adjusted to minimise blemishes.

Various methods have been proposed for compensating for variations inresponse in telecine and television cameras. BBC Research report BBC RD1985/3 discloses a system which compensates for variations insensitivity in individual elements of a line array CCD sensor. This isknown as a "stripe stripper" and operates by measuring the response ofeach element of the 1024 element line array. In effect, the systemmultiplies the output of each element by a correction factor derived forthat element to give a substantially uniform output across the array andto eliminate the vertical stripes which result from variations inresponse across the array.

GB 2149260 (Marconi) discloses another CCD compensation system. Theprinciple is similar to the proposal of BBC RD 1985/3 but applied toarea array CCDs. Thus, a correction factor is derived for each elementof array.

GB 2074416 (Ampex) relates to television cameras and divides an activevideo picture into a 13 block 14 band ratio. In a setup mode the signalfrom a selected camera is sent to an A/D converter in the video signalpath. Horizontal and vertical error measurements are made in whichselected samples within blocks of successive horizontal lines and bandsof vertical lines are summed to provide horizontal and digital datawhich are then subtracted from the measured output of each block or bandto derive a correction factor.

None of the above proposals considers how to compensate for the type ofdefects which arise in flying spot telecine. Furthermore, the twodocuments which compensate for variations in sensitivity of individualCCD elements do not consider how to compensate for other defects whicharise in the optical path, for example uneven film illumination.

SUMMARY OF THE INVENTION

The present invention aims to provide a system which providescompensation for the type of errors which arise in flying spot telecinesystems as described above.

The invention in its various aspects is defined in the claims to whichreference now should be made.

Existing flying spot telecines require frequent and complex alignment.One feature of the present invention is a method and apparatus thatenables a self-alignment operation to be performed whenever requested.Self-alignment is an automatic process carried out by the telecine inwhich it derives correction factors and, where appropriate, blemishsignals for the whole scanning area and applies these derived values tooutput video data. Individual stored corrections are given for allpixels in the picture area and these corrections applied to the videosignal during normal operation.

In a preferred embodiment the system uses a digital scan generator whichmaps a CRT raster into a 1024×1024 digital correction address map. Alinear scan drive circuit ensures that any digital scan addresscorresponds to a precise and consistent position on the CRT face. Eachof the 1,048,576 mapping points has a corresponding 16 bit memorylocation in which is stored the appropriate shading correction value.

During the self-alignment process each of these locations is measuredunder no-film conditions and the uncorrected video is preferablyintegrated over a period of time whilst the scanning spot is movedaround within its mapping pixel area. The latter techniques have theadvantage of reducing the effects of random noise and fine CRT grainwhich would otherwise be locked in as a fixed pattern. The integratedvideo measurements are then converted into shading correction values.

During normal operation the video signals are multiplied by the currentscan pixel correction value using a digital multiplier, the input videobeing a 14 bit digital signal. In one preferred embodiment a separateand self contained circuit is provided for each of the red, green andblue video channels.

The high resolution 1024×1024 map has the advantage that irregularshading patterns can be corrected, even small area changes and fairlysharp edges such as burn errors. Similarly CRT grain, and to some extentblemishes and dirt, may be corrected.

Another feature of the invention further reduces the effects of burn. Wehave appreciated that the task of the shading corrector on burn edgesmay be facilitated by spreading out the edge of the raster so that itdoes not burn a sharp edge on the phosphor.

In one embodiment of the invention slow edges on the CRT blankingwaveforms will spread the burn edges in a similar fashion. In anotherembodiment a similar effect can be achieved by accelerating the scanwaveform at the blanking edge times. Alternatively or additionally, thescanning spot may be defocused at the blanking edge times. The threetechniques of burn reduction may be combined together.

In a further feature of the invention a constant checking is performedduring the alignment process to detect any scan pixel containing videolevels below the correctable level. Any such pixel has its memorylocation flagged to indicate that it contains a blemish or dirt. Duringnormal operation any location so flagged will be treated as invalidvideo data and will be substituted by the immediately previous videodata.

A further aspect of the invention uses the video correction factorstorage area for inserting test patterns into the video path. Thestorage area is temporarily loaded with the test pattern instead of thecorrection data and the video input is forced to unity level so that theoutput of the multiplier is the test signal. The test pattern originatesfrom a separate processor system where it may reside in ROM or begenerated from a suitable set of algorithms.

In another aspect of the invention, a high speed random access memory isproduced by a multiplexing technique in which a plurality of identicalbanks of fast RAM, e.g. four banks, are each loaded with identical dataand are read by a four phase clock which thereby permits a read datarate four times greater than that possible with a single block of RAM.Whilst this system uses four times as much RAM it is neverthelesscheaper than using a single block of faster RAM. This techniqueincreases effective RAM operating speeds whilst maintaining true randomaccess.

In a further embodiment of the invention, special effects may beproduced. This may be achieved by generating digital horizontal andvertical scan addresses, continuously comparing the address withpredetermined reference addresses delineating the border of the activepicture, applying a blanking signal when the scan address exceeds thelimits of the active picture defined by the reference addresses andmodifying the reference addresses to introduce blank areas into theactive picture.

Special effects may also be produced by generation of a brightness mapwhich may be loading into the correction factor memory in place of, orin addition to, the stored correction factors. The brightness map can beselected to create a desired picture shading, for example, an opticalvignette.

Alternately, the brightness factors, in combination with the correctionfactors may be used to compensate for irregularities in the film used,for example, differences in response across the film. A clear film isinserted in the gate after the correction factors have been derived anda set of brightness factors derived which compensate for losses causedby the film. The brightness factors are combined with the correctionfactors to produce a set of compensation factors which are then appliedto the video data.

Rather than deriving the brightness factors from clear film a set offactors may be preloaded by the operator, for example, to correspond toa slope of a given gradient across the width of the film, or to aparabola. The preloaded curve may be determined from knowncharacteristics of the film, or batch of film being used.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described in detail by way ofexample and with reference to the accompanying drawings, in which:

FIG. 1-is a schematic representation of the image scanning path of atelecine embodying the invention;

FIG. 2-shows the basic concept of video signal correction;

FIG. 3-shows a section of the working area of the CRT screen dividedinto a scan map and a shading map;

FIG. 4-illustrates the effect of a blemish on the shading map;

FIG. 5-is a block diagram showing how the shading error correctingfactors are applied to each colour signal;

FIG. 6-is a block diagram showing the sampling phase of the alignmentprocess

FIG. 7-shows the use of a further look-up memory during an alignmentprocess;

FIG. 8-shows circuitry for generation of a test pattern;

FIG. 9-is a composite block circuit diagram showing how all thefunctions of FIGS. 5 to 8 may be realised;

FIG. 10-is a more detailed block diagram of the delay unit of FIGS. 5 to9;

FIGS. 11 and 12-shows timing waveforms during actual scanning andalignment respectively:

FIG. 13 - shows schematically the configuration and accessing of thecorrection factor memory;

FIG. 14 - shows the memory structure of one of three boards of memory;

FIG. 15 - shows the output stage of one memory board of FIG. 14;

FIG. 16 - shows schematically a memory error corrector;

FIG. 17 - is a flow diagram of the operation of the system controller invarious modes;

FIG. 18 - is a block diagram of the alignment mode;

FIG. 19 - shows how each pass of the alignment operation is split intosub-passes.

FIGS. 20(a) and (b) show respectively a sample trace of shadingintensity and the blemish flag;

FIGS. 21(a) and (b) are similar to FIGS. 20(a) and (b) for a modifiedblemish concealment method;

FIGS. 22(a) and (b) show an unmodified and modified image illustratingthe use of colour effects;

FIGS. 23a), b) and c) illustrate the derivation of the colour effects ofFIG. 22(b); and

FIGS. 24a), b) and c) illustrate blanking edge effects.

DETAILED DESCRIPTION OF THE EMBODIMENT

In the embodiment illustrated in FIG. 1, a digital scan controller 10produces a pattern of x;y coordinate data that is converted to ananalogue signal by D to A converter 12. The analogue signal forms theinput to CRT 14 and controls the traverse of the illuminating electronbeam over the face of the CRT. Light from the CRT passes through theimage film 16 via an optical system 18 and then to three photo-electriccells 20R, 20G and 20B, one for each primary colour, via a secondoptical system 22. The second optical system 22 includes a beam splitter(not shown) for dividing the light from the CRT into the threeindividual components. Each of the photo-electric cells (PECs) 20R, G,and B produces an individual analogue signal proportional to the amountof light that falls on it. These signals are filtered through filters24R, G. B. and digitised by analogue to digital converters 26 to formthree separate digital output signals.

In practice, the RGB digital output signals are not true representationsof the image on the films as they suffer from a number of effects thatare characteristic of the analogue/optical portion of the scanning pathand which contribute towards a substantially degraded output.

Shading is the primary degrading effect and is a global variation inimage intensity caused, primarily, by a fall off in illuminationintensity towards the edges of the CRT, and also by edge cut off in thefirst and second optics 18, 22 (FIG. 1). The transmissioncharacteristics of the various dichroic mirrors and filters used in thecolour splitting stages 22 and 24 also affect the shading. The dichroicmirrors exhibit variations in performance as a function of the angle ofincident light, whereas the filters display non-uniformity oftransmission across their surfaces. The photo-electric cells 20 alsodisplay a non-uniform response across their surfaces.

The factors mentioned above can cause variations in tube intensity of upto 50% in a random profile from edge to edge across the screen. Shadinghas a different effect on each colour channel, partly due to the CRT 14and partly due to the dichroic elements of the second optics system 22.Non-uniformity of transmission is a particular problem with bluefilters.

Burn effects caused by the scanning spot damaging the CRT face also giverise to global intensity variations. The effect is inherently random innature and can contribute to a further 20% loss of illuminationintensity. Colour dependence is largely in terms of burn amplituderather than profile.

Variations in illumination intensity caused by variations in size andorientation of phosphor grains on the CRT face are comparatively minorcompared to shading and burn losses, but nevertheless can account forvariations in intensity of around 1%.

Blemishes are exhibited as pixel-scale errors, introduced when the scanpattern encounters a defect in the phosphor surface of the CRT. Theamplitude of the effect is usually severe, giving up to 100% loss ofillumination. Because of the severity of the effect, tube specificationsgo a long way to reducing blemish problems.

Dirt on the CRT face can produce a blemish-like effect although theaffected areas are, more diffuse in nature and spread over a wider area.Dirt can, of course, occur at any point along the optical path describedbut is a particularly severe problem on the CRT face causing losses ofup to 100%, similar to blemishes.

Referring to FIG. 2, the system embodying the invention provides anautomatic shading unit which corrects all the above mentioned effectswhich cause degradation of illumination intensity and which requires nomanual setting up or adjustment.

The general approach of the system is to calculate and apply a separatecorrection factor to each colour component. This is necessary as therandom variations in perceived image intensity are, to a degree, colourdependent. Thus, in FIG. 2, the scanning path circuitry illustrated inFIG. 1 is illustrated generally at 30 and produces digital r, g, boutputs. These outputs are fed to a multiplier 32 which multiplies eachinput by an individual correction factor d, e, f to produce outputs dr,eg, fb which are corrected colour signals R, G, B. The correctionfactors d, e, f are produced by a correction unit 34 which will now bedescribed in more detail.

For scanning purposes the working area of the CRT is represented as amap, each point of which is addressable by conventional X, Ycoordinates. The working area is that area of the screen which is imagedthrough the film gate and is a square of dimensions approximatelyequivalent to 1,000 TV lines. The coordinate system takes the centre ofthe working area as the origin (. . . 0, . . . 0).

Scan resolution is 14 bits along each axis, which gives an 8192 by 8192bit grid (8K by 8K). As the intensity variations due to shading, burn,grain effects, blemishes and dirt are essentially random in nature, itis convenient for the correction factor to be derived from a storedmapping of the image area made in the open-gate (no film) condition.

The same degree of accuracy for defect mapping as scan resolution is notrequired. The scan resolution could generate a map having 64M pointswhich would be much greater than necessary. Satisfactory results can beobtained from a 10 bit resolution (1024×1024) giving 1M mapping points.Resolution below this level could degrade performance with regard to theburn and grain aspects of shading defects which vary much more rapidlyin spatial terms.

The shading map is partially illustrated in FIG. 3. The same coordinatesystem as the scanning map is used, (but at a lower resolution). Thepixels of the shading map are conveniently arranged to be bounded by themajor ordinates of the higher resolution scan grid.

It will be seen from FIG. 3 that each shading map pixel contains a 16×16scanning grid. Thus, the shading map coordinates can be derived from thescanning coordinates simply by truncating the last four (leastsignificant) bits of the scan coordinates. It should be noted that underthis system the address of the pixel in the shading map corresponds tothe scan grid point at its bottom left corner, not its centre.

Variable geometry scan patterns are employed to produce more complicatedeffects than the conventional X-Y zoom and pan, e.g. image rotation andperspective distortion. Therefore the direction of scanning will not, ingeneral, be orthogonal to the axes as shown by the unbroken scanninglines in FIG. 3.

A scan line is generated by a scan controller addressing a sequence ofcoordinate points, each point defining the centre c of a scan pixel 42.The constraints of tube resolution mean that scan and map pixels may beconsidered of similar size, although there is no common orientation orpixel registration.

The mapped shading correction applied to any scan pixel will be thatstored against the map pixel which encloses its (centre) coordinate,i.e. the map pixel addressed by the truncated form of the scan address.As may be seen from FIG. 3, most scan pixels will, in general, fall intoseveral map pixels. However, the selected map pixel will always containthe largest single "area segment" of the scan pixel. Given therelatively small change in shading value over one or two pixels there isno need to interpolate values between adjacent map pixels and thismethod of addressing is perfectly adequate. Thus in FIG. 3, scan pixels42a, 42b and 42c are corrected according to data stored against thefirst three shading map pixels of the upper of the two rows of shadingmap pixels illustrated, whereas, corrections applied to the further scanmap pixel 42d are derived from the fourth shading map pixel of the lowerof the two rows (pixel 44).

Blemishes in the tube face, and small dirt particles present a differentproblem. The relatively small size of the blemished area, in relation toscan line width, means that only scan lines passing very close to, ordirectly over, a blemish are affected.

Adequate mapping of these effects would require a much higher resolutionthan that adopted for shading; probably the full 14 bit resolution. Asindicated, such resolution is not feasible.

The problems may be overcome by flagging any shading map pixel thatencloses a scanning coordinate affected by a blemish. Referring to FIG.4, the shading map pixel having the coordinates (. . . 001. . . . 001)encloses a scanning pixel coordinate, which pixel contains a blemish.The map pixel is therefore marked as blemished.

This technique allows even the smallest blemish to be represented by alow resolution pattern of map pixels. If a scan pixel falls on one ofthe indicated map pixels it is processed for blemish correction ratherthan shading as will be described later. A consequence of thissimplification mechanism is that blemish processing will, in some cases,be applied to scan pixels not actually affected by a blemish. However, ahard-switched "blemish correction off" facility is provided for use ife.g. spurious blemish correction degrades pictures containing a lot ofsmall detail.

The corrective factors applied by the correction unit 34 are performedon the video data for each pixel of the scanned image. As mentionedpreviously, a separate correction factor is derived for each of thethree colour channels to neutralise the effects of colour-dependence inthe defects. Alternatively, correction factors may be derived for theluminance (Y) and colour difference (i.e. R-Y and B-Y) components of aconventional video signal.

The shading correcting factors are derived as follows:

The A-D coverter 26 at the output of the scan path provides a 14 bitbinary signal scaled from 0000 to 3FFF (Hex). It is assumed that 0000corresponds to (peak) "black", and 3FFF represents peak "white". Thevideo level obtained from each scan pixel is multiplied by a correctionfactor (CF) before being passed to subsequent stages of the system, suchthat:

    VIDEO OUT=CF×VIDEO IN

Correction factors are derived from the video output with an open gate,and are calculated to give the maximum possible output when multipliedthrough, that is, with no film in position. Each pixel of the shadingmap has its own, individually calculated CF.

The correction requirement is defined, for a given pixel as VOG×CF=VMAXwhere:

VMAX=maximum A-D output, and

VOG=video level-(open gate); thus,

the correction factor CF=VMAX/VOG

VMAX may be obtained from a measurement of the peak value for each pixelwith suitable gain added.

In practice, VOG is first calculated as an average value for each mappixel, obtained over 64 separate scan passes, so that temporal averagingwill reduce the effects of noise and glitches on CF. The individualsamples are also distributed spatially within the bounds of the mappixel, again for averaging purposes, and to give better area coveragefor picking up small blemishes. Obviously it is highly undesirable to"correct" a video signal using an erroneous correction factor generatedfrom a noise spike.

The range of CF is restricted such that shading correction is normallyonly applied to those pixels producing an average open gate output atleast equal to 30% of VMAX. Where this threshold is not achieved, thepixel is assumed to be blemished, a blemish correction is applied aswill be described. A CF value is still calculated in this case, for useif blemish processing is turned off, but it is clipped at a maximumvalue of 3.99.

Next, the blemish corrections are applied. When a video signaloriginates from a pixel identified as blemished, no multiplicativecorrection is applied. Instead, the most recent "unblemished" VIDEO OUTvalue is repeated. (Neither VIDEO IN or CF can be relied upon 100%,depending upon how badly the pixel is affected by the blemish and thealignment of the scan line).

Blemishes are identified by either of two criteria. The first of theseis a 30% threshold of VMAX (as previously described). The second is alower absolute blemish threshold in which a blemish is assumed if any ofthe 64 values of VOG obtained from a pixel during the averaging processis less than the threshold value-(VMBLEM), value. The second qualifyingcriteria is necessary to pick up shading map pixels on the edge ofblemishes which may not be sufficiently affected overall to fail theaverage level test. VBLEM is set sufficiently below the 30% threshold toavoid spurious blemish indications being given by noise on the videosignal.

If the blemish processing mechanism is turned off, the shadingcorrection factor CF is again applied to each pixel.

FIG. 5 illustrates, schematically, the hardware required to process anincoming video signal to apply the correction factor CF and blemishcorrection to each pixel. A separate circuit is used for each colourchannel or each video signal component.

The incoming scan coordinate x, y is used as a memory address to alook-up table 50 of stored CF values which provides one input to thevideo multiplier 32 (FIG. 2) at d, e or f.

As mentioned previously, the the CF value is derived in the open gatecondition. The address to the look up table 50 is delayed in a delay 52in order to give the correct matching between scan coordinate and videopixel so that each pixel is multiplied by the correct CF. The delay isequal to differences in propagation time through the scanning path 30 ofFIG. 2 and correction or shading unit 34.

The look up table 50 has two outputs, the output 54 corresponding to themultiplier 32 in FIG. 2 and a second output to a hold circuit 56. Thehold 56 prevents the output from the multiplier when a blemish isindicated and instead substitutes the value of the most recentblemish-free output.

The correct functioning of the look-up table depends on it beingcorrectly set up. The table must be aligned at various intervals; whenthe machine is first turned on and at other times thereafter as desired.During the alignment process the correction or auto shading unit 34operates to derive the correction factors as previously explained. Thecircuitry necessary for the derivation will now be described.

The alignment process comprises two stages: video sampling andcorrection factor generation. During the sampling stage an average valueof VOG is derived for each pixel, and each pixel is checked forblemishes. In the second stage, the actual correction factors arecalculated, based on the data obtained from sampling. This lattercalculation is as previously described.

FIG. 6 shows a block diagram of the video sampling circuitry. Thecircuit again is individual to one of the R,G,B signals. Alternativelyindividual circuits could be provided for Y and colour differencesignals. The delay 52 and look-up memory 50 are the same as that of FIG.5.

When the input video is being sampled, the look-up table is used as atemporary storage area for intermediate data values and blemish values.The table is operated in a read-modify-write mode. The incoming videodata value for each pixel is added to the intermediate sum for thatpixel, I-SUM, in adder 56. The resultant sum output of adder 56 SUM iswritten back in the look-up table at the same location. This process isperformed 64 times for each pixel location. However, on the first pass,I-SUM is necessarily forced to zero.

The operation may be expressed mathematically as follows:

First Pass: SUM=O+VIN (1)

where VIN (1)=VIDEO IN (Pass 1) etc

Subsequent Passes: SUM=I-SUM+VIN (n)

where I SUM=VIN (n-1)+VIN (n-2)+. . . +VIN (1)

until SUM=I SUM+VIN (64)

then SUM=VIN (av)×64

A blemish logger 58 is connected in parallel with the adder 56 and logseach input value against the single-sample blemish threshold ie. thesecond blemish criteria mentioned previously. If the sample is less thanthe threshold value a BLEMISH signal is outputted from the blemishlogger 58 and stored at the current memory location in look-up table 50with the current sum. Similarly to the sum calculation, the output fromthe look-up table as the first pass is forced to zero so that existingvalues are ignored.

On subsequent passes the stored condition (if any) of BLEMISH is readback to the blemish logger as a signal PB (Previous Blemish). If PB isset, BLEMISH is re-asserted regardless of VIN. This function isnecessary because a blemish signal must be asserted if there is ablemish anywhere in the particular pixel. As the individual passesmeasure VIN at different points in the pixel, not all passes will pickup a blemish. Thus, a single blemish occurrence is effectively latchedin for all the remaining passes in the averaging sequence.

FIG. 7 shows how the look-up table is loaded once the sampling processhas been completed. The look-up table is loaded with CF values takenfrom a programmable read only memory PROM 60 which is itself coded as alook-up table. Data is output as VOG (av) and BLEMISH signals. Theaverage open gate video level VOG (av) is derived from the stored sum ofsixty four values by truncating the five least significant bits of thesum, which is equivalent to a divide by sixty four operation. This valueis used to address the PROM 60, together with the final indication fromBLEMISH (either a positive "blemish detected" as a negative "no blemishdetected" signal). The appropriate correction factor is then stored inthe main look-up table.

FIG. 8 shows how the look-up table 50 may be used to provide a systemtesting facility.

Instead of (or in some cases as well as) loading the look-up memory 50with correction data the correction factor table is directly loaded withdata forming a video test pattern. Input of this data may be controlledby the system microprocessor (not shown). The test pattern may besupplied from a store or be produced according to an algorithm directlyfrom the microprocessor.

With the look-up memory so loaded, the output video signals are readfrom the look-up area in response to a scan coordinate, rather thanderived from the video input, which for most purposes is ignored. Thetest facility operates in two modes --low resolution pattern and highresolution pattern.

In the low resolution mode, horizontal pattern changes are restricted toevery fourth pixel. There is no need to repeat the full alignmentprocess at the end of the test run before resuming normal operation asthe correction factors CF need not be removed from the look-up memory50. In the low resolution mode only part of the table area is used andall the normal contents are preserved. In the high resolution modehorizontal pattern changes occur in every successive pixel. The amountof look-up area required is such that all stored CF data is destroyedand the entire alignment process must be re-run before normal operationcan be undertaken again.

Thus, in FIG. 7, the multiplier 53 (illustrated in FIG. 5) is loadedwith a constant ×1 input and the output from the look-up table (the testpattern). The output of the multiplier is therefore the test patternmultiplied by unity.

In practice, the components illustrated in, and described separatelywith respect to FIGS. 5 to 8 are combined on a single processing board,there being one such board for each colour signal R, G and B. Theoverall configuration is illustrated in FIG. 9. No further descriptionof the previously mentioned components is necessary. However, it will benoted that refresh counter 70, bus interface 72 and error corrector 74have not been described. The refresh counter and error corrector arenecessary purely to maintain the integrity of the data stored in the CFmemory, which in this embodiment is implemented in dynamic RAM.

Communication with the rack control bus is handled by the bus interfaceblock, which allows data transfers to/from the memory area and theoperating registers of the system controller 76. The latter, shown inisolation in FIG. 9 for convenience is primarily concerned with thecorrect sequencing of data into and out of the CF memory duringoperation.

It will be noted from FIG. 9 that common data buses 78 and 80 are usedfor data in, data out communication between the look-up memory 50 andthe interface 72, error corrector 74, correction PROM 60, blemish logger58, adder 56 and multiplier 53.

The scan coordinate data has been shown passing through a delay 50 inFIGS. 5 to 9. This block, otherwise referred to as the scan interface,is actually rather more complicated than a simple delay line and isshown in schematic form in FIG. 10.

The input signal comprising a 20-bit coordinate and associated clock andblanking signals, are as follows:

SCAN CLK (81) Continuous, pixel-rate (18.375 MHz for 625 line, 18.3776MHz for 525-line), scan coordinate clock signal. Nominally square inform.

LINE BLANKING (82) Continouous line-blanking waveform, maintainedregardless of field timing. Also referred to as "LB".

BLANK (84) Composite, line-and-field, video blanking.

FIELD START (88) A single pulse during LB to indicate the start of avideo field. Also referred to as "FS".

The first signals are shown in diagramatic form in FIG. 11 together withthe scan coordinate data stream. It should be noted that the lineblanking signal LB is active during field blanking to maintain thememory refresh processes of the shading unit. Actual blanking of thevideo output from the shading board is achieved using the compositesignal BLANK.

During the alignment process previously described, the scan pattern iscompressed such that the field blanking period is the same length as theline blanking period. In this case, LB and BLANK are identical. This isshown in FIG. 12 which also shows the positioning of the field-startpulse 88.

All the scan signals are received in ECL differential form from anotherrack. They are converted to conventional, single-ended TTL format andthen de-skewed with respect to SCAN CLOCK by a series combination of atransparent latch 90, driven from a pulse stretcher 92 and anedge-clocked register 94. This gives a clean stream of synchronous data.

Unfortunately, because these signals originate from another rack, thereis no guaranteed phase relationship between SCAN CLOCK and the SYSTEMCLOCK used throughout the shading unit. Changing of data synchronisationfrom one clock to the other is done via an intermediate register 98driven from a delayed form of SCAN CLOCK. The delay shown as block 96 is(at least initially), arranged to be adjustable, so that any degree ofphase offset can be accommodated.

A variable-depth pipeline stage 78 driven by the system clock splits thescan signals into two groups and outputs them to the rest of thecircuit. The full coordinate address is fed via tri-stateable outputs tothe main address bus connecting to the memory block. A second,permanent, output routes the blanking signals and the fourleast-significant bits of the (horizontal) coordinate address to theshading controller two cycles in advance of the main address output.This is to allow the controller time to process the information beforethe associated coordinate appears as an address at the input of thememory.

The overall depth of the pipeline is set, in conjunction with a similarpipeline in the scan controller, to cancel out the propogation delays ofthe scanning path as mentioned previously achieved by keeping theaddress input to the memory sufficiently ahead of the video data streamso that, the CF read from memory reaches the video multiplier, at thesame time as the data to which it applies.

It has been realised that one of the factors that affect burn is fastedges on CRT waveforms as they produce sharp burn edges. In order toassist the shading corrector, burn damage can be reduced by utilisingslow edges in the CRT blanking waveforms to spread the burn edges, or byaccelerating the scan waveform at the blanking edge times. The spotcould be defocused at the blanking edge times to reduce intensity.However, care must be taken to avoid extra burning within the unblankedarea near to the blanking edges. The signals shown in FIGS. 11 and 12are purely schematic. In practice, the edges of the blanking waveformsare spread over several clock cycles. A combination of these techniquesmay be used.

The look-up memory has until now been considered as a block memory 50.It was mentioned with respect to FIG. 9 that the memory was implementedby dynamic RAM with the appropriate refresh and error correctingcircuiting. The memory will now be described in greater detail withreference to FIGS. 13 to 15.

The design of the correction factor memory is a function of three basicparameters; the amount of storage required, the data width, and thememory cycle time. The minimum amount of storage is, of course, onelocation per map entry, conveniently giving full utilisation of a 1Mdeep RAM area which corresponds to the shading map resolution.

Data width is determined by the range of correction factors to be used,and the bit-resolution to be maintained. Correcting open gateintensities as low as 30% of maximum requires a CF range from 1.00 to3.33. Normalising the video input to 1, a 16-bit correction factor isnecessary to preserve information from all bits of the video input. Thisallows an actual range extending to 3.99 (4 less one LSB), which is thatused when a blemish is located. The memory area is accordinglyconfigured to provide a data width of sixteen bits, plus an additional(17th) bit for parity.

No separate provision is made for the storage of blemish data. Instead,blemish indication is coded into the correction factor. The restrictedrange of CF means that the two most-significant bits will never be 00for a valid correction factor (none are less than 1). These two bits canbe used as a means of blemish coding. If a location is judged to beblemished, the sequence of bits in the correction factor is altered tomove the two MSBs to the LS end, and the first two bits at the MS endare set to 00. Thus, when operating normally, any CF read as 00 . . . isignored and blemish action initiated. If blemish processing is turnedoff, the CF can be reconstructed by reversing the MS and LS bit pairs,although two bits of resolution are lost in this case.

A wider memory area is needed for temporary data storage duringalignment. In particular, summing sixty-four 14-bit video levels duringthe sampling process needs a 20 bit data width to preserve informationfrom the LSBs. Since this extra width is only needed during therelatively short alignment period, it is provided by temporaryre-arrangement of the normal 16-bit memory, rather than expanding thewhole area to 20 bits.

The major difficulty in this area is the rate at which data has to beread from the memory. To keep up with a stream of video data having asample rate of 18.375 MHz it is necessary to read a new CF every 54 ns.Additionally, the unpredictable scanning sequences used in effectsgeneration means that access to the CF store has to be truly random.Each CF must therefore be read in an individual, discrete, access cycle.This in turn dictates that the effective access time of the memory areamust be somewhat less than 54 ns, allowing for propogation delays, etc.

Achieving this order of performance in a simple, single-block memory isonly possible using high-speed static RAMs. Unfortunately, these areprohibitively expensive in the quantities needed here. We haveappreciated that the problem may be overcome by using duplicated areasof slower memory and accessing them on different timing phases toachieve the desired overall data rate. Such a configuration is in FIG.13.

FIG. 13 shows the basic idea using a four-plane memory arrangementmemories A, B, C, D. Incoming data requests (addresses) are directed, ina repeating A-B-C-D sequence, to different memory planes. The outputdata is similarly drawn from each plane in turn, after allowing a periodfor data access. Individual memory planes are now addressed at onlyone-quarter of the request rate, i.e. the available ACCESS CYCLE is nowfour-times the DATA CYCLE.

Even in such an arrangement of low-speed devices, static RAMs areunsatisfactory on grounds of cost and/or packing density. (Low-speed,static RAM is only cheap in low-density, large-package form). DynamicRAM is substantially cheaper but suffers from the problem that itsrandom access cycle time is typically three times the usually quoted"address" access time. As previously mentioned the present system relieson a completely random address sequence due to the uncertain andchanging relationship between scan waveform position and shading map.However, we have found that there are 80 ns devices that will providethe desired performance in the proposed four-plane system.

FIG. 14 shows for one colour only the arrangement of the memory in fourparallel planes A, B, C and D. Each plane is identical and comprisesseventeen 1M×1 DRAMs 100, with latched address inputs 102. In all, threeidentical boards are provided, one for each component of the videosignal. Dynamic RAMs are addressed as a square, row-column matrix, withthe row and column addresses separately input on one set of addresspins. This allows a substantial reduction in pin count, and hencepackage size. (Here, only 10 address pins are required to input a 20-bitaddress). However, an external multiplexing arrangement is needed whenoperating from a full-width address bus. In this case, the addressmultiplexing function is provided by the address latches, which aresplit into two parts with separate output-enables (OE).

All four planes are connected to a single address bus, while all othersignals are unique to each plane. There are then, four address clocks,four RAS signals, etc., etc. Data cycles are routed via different memoryplanes by appropriate sequencing of the various control signals.

Data from the individual memory planes is combined into a continuousstream of CFs by an arrangement of latches 104-110 shown in FIG. 15.Each latch is enabled (EN) in sequence to accept data from its ownmemory plane, and then turned on to drive the combined bus, normally forone data cycle only. (This timing restriction does not apply whenperforming slow-time reads via the bus interface). The CF data ischecked for valid parity before being clocked through another latch. Inthe event of a parity error, the latch is disabled, causing thepreceding (valid) CF to be re-used.

This error handling mechanism is only used on-the-run and faults are notflagged to the main error corrector, which functions autonomously andwill detect and process any errors for itself.

The output of the latch 104-110 is fed directly to one input of thevideo multiplier 32 (FIG. 2) with the exception of the LS and MSbit-pairs which are taken through a decoding circuit 112 first. Thisimplements the blemish processing function, if it is beingused,-outputting a hold signal to the multiplier when a blemish code isdetected (corresponding to block 54, in FIG. 5). It also provides forced"times 1" and "times 0" functions, as well as video blanking, by directdriving its output lines to the appropriate conditions, while at thesame time forcing the latch outputs to zero.

A feature of dynamic RAMs is that they are susceptible to soft errorscaused by the impact of alpha particles. Quoted error rates aretypically in the order of 0.01% per thousand hours, although moderndevices could well achieve better than this. (Error rate data is notreadily available). This error rate looks fairly insignificant until itis considered in context.

The working area of each memory plane (ignoring parity), is 1M×16, or16,777,216 bits. Together, the four planes total 67,108,864 bits. Whenapplied to this many bits, 0.01% per thousand hours is equivalent to onebit error occurring, somewhere in memory, on average every nine minutes.Such errors are randomly distributed and could easily occur in one ofthe more significant bits of the CF, causing a visible defect in thedisplayed image. As the equipment is usually operated for long periodsof time, there is the opportunity for such errors to accumulate, givingincreased image degradation.

The multi-plane architecture adopted for the memory has the advantagethat it provides an easy means of correcting data errors as part of thenormal dynamic memory refresh process, which is here conducted duringline blanking so as not to disrupt the video image.

Instead of using a simple RAS-only refresh mechanism, cycling throughthe required 512 refresh addresses, refresh is achieved in the course ofcontinually rewriting all the data stored in the memory. Aread-modify-write mechanism is used, with all four planes acting inphase with each other.

Corrected refresh data is generated by taking a bit-wise majority voteof the data read from planes A, B and C as shown in FIG. 16. This isthen written back into all four planes. Unlike correction mechanisms forsingle-plane memories, there is no overhead of memory needed foradditional error coding bits.

Such a process of combined refresh and error correction is referred toas error scrubbing. It is of course necessary to individually scrubevery memory location. A full scrub cycle takes a lot longer than theusual 512-address refresh cycle, which is, still present as a sub-set ofthe scrub cycle. Restricting refresh access to the line blanking periodsallows seven locations to be refreshed in each line period. At thenormal line frequency of 21 kHz, refresh is achieved in 3.48 ms, and afull scrub cycle completed in 7.1 s. To avoid any possibility of errorsappearing as glitches before they are scrubbed away, the paritymechanism described with reference to FIG. 15 used to temporarilynullify unscrubbed errors by repeating the preceding un-errored, CF.Even at the minimum scanning rate, no mapped CF would be affected bycorrective action from the parity circuit for more than about 11seconds.

During alignment, it is particularly important that no errors areintroduced, since a corrupted CF could subsequently give rise to apermanent pixel error in the video picture. A three plane arrangement isused, with data being written simultaneously into planes A, B and C,each being extended in width using one-third of plane D. Whenintermediate data is fed to the adder and blemish logger, or thecorrection factor generator, this is done via the error corrector,--oneoutput of which is permanently routed to the inputs of those stages.

The system controller 76 shown in FIG. 9 retains overall control of alltransfers of data into and out of the CF store 50. It also generatessignals affecting the operation of the video processing stages. Althoughthe majority of functions of the controller are not relevant to thepresent invention, it is desirable to consider how the controllerbehaves during the alignment phase.

FIG. 17 is a flow chart showing the operation of the system controller.

The controller can operate in four modes: Run (normal operation); BusAccess (to allow the system bus access to the main memory area duringperiods in which video processing is suspended); Reset (entered intoafter power-up or a forced reset); and Alignment Mode. As can bedetermined from the arrows linking the individual sections of FIG. 17,mode switching is limited. In the reset mode the system can move intoeither the alignment (shading map derivation) or bus access modes 150,152. From the Alignment mode 150 the controller may move to either thebus access mode 152 or the run mode 156 and from the bus access modeonly the alignment and run modes may be entered. From the run modeeither the alignment or bus access modes may be entered. The reset mode154 may be entered by a `hard` request.

In the alignment mode, a self-regulating process intensity maps the faceof the CRT and produces a combined correction factor/blemish map in theCF memory 50--this is the alignment operation already described. Apartfrom initiation (with an open gate), the only external requirement isthat the scan controller should be switched to continuously scan thefull CRT area.

FIG. 18 shows in block form the alignment sequence previouslydescribed-there are three basic functions; an initial sampling pass 114,63 further passes where samples are summed with the existing data 116,and finally a pass 118 where the CFs are produced. The areas of the flowchart of FIG. 17 corresponding to these blocks are illustrated on thatfigure.

Alignment is performed with the scan pattern running at full speed, i.e.a new pixel address every 54 ns. This is necessary to ensure thatafterglow effects experienced are representative of those during normaloperation. Scanning in slow-time would result in a shading map that isunusable since afterglow effects are dependent on scan speed.Unfortunately, the data processing for each pixel takes a lot longerthan the 54 ns cycle time. The adder alone requires two cycles toproduce a result, and the memory planes are being used in-parallel inread-modify-write mode which also slows the process significantly. Toget round this problem, each alignment pass is performed in a series ofsixteen sub-passes, with each sub-pass spread over one complete videofield. Pixels are sampled on a one-in-sixteen basis, thus allowingfifteen clock cycles between consecutive samples to complete the variousdata transfers and additions. This process is illustrated in FIG. 19.

At this stage a comparator on the output of the sub-pass counter isused. A pixel is only sampled if the four least significant bits of itsaddress match with the current value of the sub-pass counter, whichcycles from 0 (1st sub-pass) to 15 over the sixteen fields of a pass.Thus, after all sixteen sub-passes have been completed, every pixellocation in the scan pattern has been sampled once, and once only.

Referring back now to the area of FIG. 17 enclosed by the chain dottedline indicated by 114, on entering the alignment mode, no action istaken until the start-of-field signal (FS) is encountered (WAIT),whereupon the first pass is started. this delay over-rides normal memoryrefresh, but any data loss so caused is of no consequence since theentire RAM contents are going to be redefined anyway. The pass andsub-pass counters are set to zero, so the comparator circuit gives anaddress match (MATCH) every 0'th pixel. Immediately line blanking (L.B)ends, pixel sampling begins. This process is interrupted at the end ofeach line while a number of refresh cycles are executed during theblanking period. At step 160, SCRUB, three memory planes operate inparallel and the fourth is split between them. Corrective scrubbing isperformed at this point.

No further changes take place until an FS pulse is detected during lineblanking. This indicates that one complete field has been processed andthe next is about to begin. The sub-pass counter is incremented so as tosample the next pixel along and the whole process is repeated. When allsixteen sub-passes have been completed, this pass is over, and controlexits to the next stage of alignment (Box 116).

Flow control during the summing passes is basically the same as for theinitial pass, excepting that at the end of each pass the pass counter isincremented before processing is resumed. Exit from this stage isdetermined by the pass counter reaching its terminal value.

The final pass 118 proceeds in similar fashion to the others. Here"convert" step 162 denotes the conversion of summed video data into a CFvalue. An important point to note is the change in refresh format. Nowonly the three parallel planes are scrubbed, the other, plane D, isrefreshed without any rewriting of data. The reason for this is thatduring this pass, plane D usage is progressively changed from split3-layer to normal single-plane,-as raw data is converted to CFs. Ifscubbing were applied to plane D as previously, any CF values scrubbedover would be corrupted.

Once the final pass is completed the process sits in a "wait" conditionuntil instructed to proceed elsewhere. Refreshing continues to beinitiated by the line blanking signal, all four planes being scrubbed inparallel now that CF conversion is complete and the entire contents ofplane D are in single-plane format.

The bus access mode 152 allows the system bus access to the main memoryarea and video processing is suspended in this mode. The controllerwaits for, and processes, read and write transfer requests, datatransfers being handled in two stages via intermediate registers in thebus interface. Memory refresh continues (steps 170, 172, 174), the exactrefresh operation depending on the current run mode.

Distribution of data transfers between memory planes is controlled byBank select registers 176, 178. These registers provide separate readand write select functions. Data can be read from any one memory planeand write operations can be to a single plane, planes A, B and C (at180) or to all planes (at 182).

The run mode 156 has three operating conditions: high resolution test184, low resolution test 186 and normal video processing 188.

As the low resolution mode uses only one memory plane to store its testpattern the other three planes store their original data. This conditionis maintained at refresh by scrubbing 3 planes and treating the oneplane in isolation. Transition between the three run modes wouldnormally be made via bus access or alignment modes.

FIGS. 20 and 21 illustrate how the blemish detection and concealmenttechnique described with respect to FIGS. 9 may be modified.

Although the technique works satisfactorily, it has been found to havetwo significant drawbacks. Firstly, samples around the edge of a blemishtend to have a shading value which is higher or lower than the averageshading value in that area. This difference is due to take effects whichappear around the blemish and to difficulties in defining the blemishedge. The effect of the halo is that into the blemished area issubstituted data which is not representative of the average shadingvalue of the area and so the blemish is only partially concealed.Secondly, small blemishes are difficult to detect as the signal may bereduced by a small amount only. That amount may be comparable to shadingand burn errors and so a small blemish may be misinterpreted.

The modified system overcomes these two disadvantages by post processingof the contents of the correction signal memory.

The memory contents are read by a microprocessor which comparessuccessive samples. If two successive samples differ by more than apredetermined threshold value, then both the samples are flagged asblemished. The original level dependent threshold values are alsomaintained. Thus, where one sample is representative of the actualblemish, both it and the neighbouring sample, representative of thehalo, will be flagged as blemished.

The flagged area is enlarged with respect to the earlier embodiment.This is illustrated by FIGS. 20 and 21. In FIG. 20, which shows theflagging of a blemish according to the earlier embodiment, the width ofthe blemish flag pulse 120 is equal to the duration for which the signal122 falls below the blemish threshold level 124. Thus, broken lines 126and 128 show the desired unblemished signal, correcting blemish 130 andsmall blemish 132. However, broken line 134 shows the only correctionthat is made as the value of the sample adjacent to the blemished areabelow the threshold value is very much lower than the unblemished value.Even if the blemish well was steeper, the adjacent sample would beinaccurate as it would represent the halo around the blemish indicatedby peaks 136 and 138 in the signal either side of the blemish.

The modified blemish signal obtained by using the post processing methoddescribed is shown in FIG. 21(b). As mentioned previously the methoduses rate of change thresholds rather than shading level thresholds.This means that both the halo around the blemish and the blemish itselfwill be flagged as blemished as comparison of adjacent shading valueswill produce a difference greater than the threshold values. Moreover,the `depth` of the blemish, that is its lowest shading value, isunimportant as the method identifies blemished areas by the slope of thewalls of a blemish well. This means that smaller blemishes such asblemish 132 of FIGS. 20(a) and 21(a) will be corrected.

In practice the blemish flag will be two samples wider than the actualblemish. As mentioned, where two samples differ by an amount greaterthan the threshold value, both are flagged as blemished. This means thatat the leading and trailing ends of a blemish the last unblemishedsample and the first blemished sample will be flagged. Thus, the blemishflag is one sample wider at each end than the blemish. This; means thatthe sample adjacent to the blemish flag will always be unblemished sothat the corrected signal with the blemish concealed shown by brokenlines 140 and 142 in FIG. 21(a) are almost identical to the unblemishedsignal 126 and 128 in FIG. 20(a).

From the discussion of FIGS. 20 and 21 it will be appreciated that thefirst mentioned disadvantages is avoided as the blemish area is widened.The second disadvantage may also be avoided as the comparison usedeffectively detects the edges of a blemish. This means that thethreshold value may be set very low which enables a betterdiscrimination to be made between blemishes and burn or shading defects.

To implement the modification, the microprocessor reads a small area ofthe shading memory, compares adjacent pixels in the horizontal directionand stores the location of any blemishes. A read-modify-write operationis performed on the blemished locations to add a blemish flag to theshading memory contents. Of course, adjacent pixels in the verticaldirection could also be compared. However, this would require moreprocessing twice during the alignment operation.

It should be appreciated that both the blemish detection and concealmenttechniques described could be used together in a telecine system.

In further modification the shading correction map may be utilised toproduce a wide range of special effects. Although it is realised thatthese effects are in themselves known, it has until now been necessaryto use specialised digital picture effects equipment to produce theeffects. This modification enables the effects to be produced by shadingcorrection circuitry and thus by a telecine machine.

In a first aspect of the modification the shading correction circuitrycan be used to provide predetermined spatial variations in thebrightness of the telecine picture signal. This may be achieved byloading into the shading correction memory 50 (e.g. FIG. 6) apredetermined brightness map, either instead of or in addition to thecorrection factors. As the output video data VIDEO OUT is the product ofthe input data VIDEO IN and the contents of the correction factor memory50, the output video is a signal modulated by the brightness map. As anexample the shading memory may be loaded with data representing fullgain for an ellipse centred on the picture centre, and graduallyreducing gain towards the edges of the picture. The effect produced willbe similar to an optical vignette. It will be realised that any desiredbrightness effect can be produced simply by loading the correctionfactor memory with the desired brightness map.

In addition, parts of the picture may be masked completely by reducingthe gain of those parts to zero.

The brightness maps may either be stored in look-up tables or generatedby algorithms and are loaded into the correction factor memory 50 via amicroprocessor.

The brightness factors, as well as being used to produce specialeffects, can be used to compensate for defects in the film. It is commonfor the sensitivity of film to vary across the width of the film and fordifferent batches of film to exhibit different characteristics. Bypredetermining the response of a given batch of film a set of brightnessfactors can be derived which correspond to a given correction curve, forexample a slope or a parabola. This curve may be derived by insertingclear film into the telecine and comparing the difference in response inthis condition with the no-film film condition.

To compensate for the film condition the selected brightness factors aremultiplied with the stored correction factors to give an overallcompensation factor for each pixel of the scanning map. Thesecompensation factors are then applied to the respective pixels of videodata as previously described.

It will be appreciated that coloured effects may also be produced asthere are three separate sets of correction factors generated in threedifferent memories, one for each of the red, blue and green channels.

FIGS. 22 and 23 show how colour effects may be produced and alsoillustrate how the brightness effect just described may be produced.

FIG. 22a shows an image to be scanned, in this case a push buttontelephone, and FIG. 22b shows the final effect produced by colourmixing. The effect is very simple, but is for illustrative purposesonly. It will be understood that much more complex effects may beproduced if desired. In FIG. 22b the picture has been divided intoquadrants, the top left quadrant being shaded red, the bottom left,yellow, the bottom right, green and the top right, blank.

The formation of the quadrants can be understood from the individual R,G, B shading maps shown in FIGS. 23a, b and c.

FIG. 23(a) shows the shading map for the Red signal. The processor loadsthe correction map for the R signal such that the left side of the imagehas maximum output and the right side has zero output. Similarly in FIG.23(b) the correction factor map is loaded such that the bottom half ofthe map is maximum green output and the top half zero output. Thecorrection factor map for the blue signal is loaded with zeros for allmemory locations which inhibits the blue signal.

The three shading maps are superimposed to give the colour tinteddisplayed image of FIG. 22(b). In the bottom left quadrant the red andgreen halves overlap to produce a yellow quadrant. In the top rightquadrant the correction factor maps are loaded with zeros for all threecases so that the top right hand quadrant of the displayed image isblank.

It will be appreciated that the brightness effect described previouslycan be produced by loading a brightness map into memories of each of theR G B boards. Thus for example if the pattern of FIG. 23(a) was loadedinto all three correction factor memories the resulting image would beat maximum brightness on the left hand side of the image and at minimumbrightness on the right hand side of the image.

The brightness modulated picture is, of course, uncorrected. To retainthe function of the shading corrector the correction factor must be readfrom each location, the value at that location modified accordingly, andthe correction factor returned to the memory location.

It will be appreciated that the brightness effects will take asignificant amount of processing time to produce and remove.

To compensate for this, additional shading memories 50 may be includedwhich can be loaded slowly and switched into operation when required.

In a second aspect of the modification a film image may be modulated ormixed with another image. It will be remembered that the correctionfactors in the method as initially described were derived with thetelecine in an open-gate (no-film) condition. To mix images the pictureto be mixed is introduced into the telecine film gate during shadingalignment. The effect of this is that the correction factors derived andstored in memory 50 are the product of the shading correction factorsand the inverse of the picture in the film gate.

Alternatively, pictures may be mixed by altering the shading correctionroutine so that the existing correction factors for each location arefirst read from memory 50, multiplied by the picture image at thatlocation and the product returned to the shading memory 50. Thus, astored picture image and correction factor results so that it is thisproduct which modulates the subsequent film images scanned by thetelecine rather than the correction factors alone.

A third method of loading pictures into the shading memory would be totransfer directly to the memory data from a digital picture store orother picture source. This data may be modified using the existingstored correction factors.

Once a picture or effect has been stored in the shading memory 50 usingany of the aspects of the modification described, it may be altered oredited by interactive control from a graphics tablet or other suitableapparatus using a microprocessor control system.

Although parts of the picture may be masked or blanked by reducing thegain to zero for that area is mentioned previously, a different methodis preferred for convenience. This method involves altering the blankingsignal reference points such that the video signal is switched off forthe areas which it is desired to blank.

FIG. 24 shows a further effect that may be generated. The scangeneration circuitry produces a digital address for all horizontal andvertical scan positions. These addresses are compared with fourreference number pairs, R1, R2, R3, R4 (FIG. 24a) by a blanking signalgenerator. The reference numbers correspond to the top left, right andbottom left, right positions of the picture. Whenever the pictureexceeds these limits a blanking signal is produced which is applied tothe video amplifiers to turn off the video signal. Effects are thenapplied to the blanking signal by modifying the reference numbers. Forexample, adding an offset to the left reference number moves theblanking edge to the left or the right depending on the sense of theoffset. Adding a part of the vertical address to the left referencenumber will slope the left edge to one side. Curved or irregular effectscan be applied to the blanking edges by modifying the data applied tothe reference numbers using look-up tables.

Thus, in FIG. 24(b), the left blanking edge is offset by increasing thehorizontal reference numbers so that R₁ and R₃ are moved to the right oftheir FIG. 24(a) position. The vertical numbers of R₃ and R₄ areincreased, raising the lower blanking edge. The result is that the imageis displayed with offset blanking edges as shown in FIG. 24(b), with theleft most and bottom most portion of the image being suppressed.

FIG. 24(c) shows the effect of adding a part of the vertical address tothe left reference number. This causes the left blanking edge to slopeto one side. In FIG. 24(c) the left blanking reference number has beenincreased also, causing R₁ to shift slightly to the right.

It should be understood that any of the effects outlined above can beprogrammed by a suitable telecine pre-programmer.

Although the description has been given in relation to a flying spottelecine, it should be appreciated that the invention in all its aspectsis applicable to the situation where the telecine is used as a filmwriter. That is, where unexposed film is introduced in to the film gateand the video signal is split into three separate components; R,G,B eachof which, in turn, is used to modulate the flying spot to write thefilm.

We claim:
 1. A cathode ray tube (CRT) flying spot image scanningapparatus, comprising:a scanning area for scanning with a flying spot;means for addressing each location of the scanning area; means forderiving a correction pixel map addressable by coordinates generated bythe addressing means; means for weighting an output video signal tocompensate for defects and losses along the optical path of theapparatus, the weighting means comprising means for dividing thescanning area into a plurality of sub-areas for corresponding to a pixelof the correction pixel map; means for determining a correction factorfor each subarea of the scanning area indicative of defects and lossesassociated with that area; storage means for holding derived correctionfactors; means for applying the correction factors to output video datafrom the apparatus to produce a weighted video signal; the addressingmeans including a digital scan generator and a scanning pixel mapderived from the digital scan generator, the orientation of the imagescanning performed by the flying spot and correction pixel map withrespect to one another being variable; and means for applying thecorrection factor derived for a given pixel to a pixel of the picturescan whose centre falls within that correction map pixel.
 2. Apparatusaccording to claim 1, comprising means for comparing the correctionfactors of adjacent pixels, means for marking as blemished any pair ofpixels whose comparison falls outside a predetermined threshold, meansfor suppressing the correction factors of any pixels marked asblemished, and means for substituting for pixels corresponding tosuppressed correction factors, video data derived from adjacent pixels,wherein the means for applying the correction factors to output videodata operates only on unsuppressed correction factors.
 3. Apparatusaccording to claim 1 or 2, comprising means for loading into the storagemeans brightness factors for each pixel of the scanning area, and meansfor modifying the input video signal by multiplying for each pixel,video data from an area of the scanning map corresponding to the pixelwith the stored brightness factor for that pixel.
 4. Apparatus accordingto claim 3, further comprising a plurality of stores for each videosignal component and means for pre-loading into each store an individualset of brightness factors.
 5. Apparatus according to claim 3, furthercomprising means for combining the brightness factors with the storedcorrection factors to provide a set of compensation factors, and meansfor combining the compensation factors with the video data from thecorresponding areas of the scanning means.
 6. Apparatus according to anyof claims 1 or 2, further comprising means for loading into the storagemeans picture information relating to a first image in addition to thecorrection factors and means for modifying a second image by multiplyingthe stored correction factors and first image data with the second imagedata for each pixel of the second image.
 7. Apparatus according to anyof claims 1 or 2, further comprising a blemish detector for detectingpixels of the correction pixel map in which the output is lower than apredetermined threshold, and means for suppressing application of acorrection factor to a given pixel on receipt of a blemish signal forthat pixel.
 8. Apparatus according to claim 7, in which the weightingmeans further comprises means for substituting for a suppressed pixel,compensated data from an adjacent pixel.
 9. Apparatus according to claim7, wherein the blemish detector includes means for detecting whether asample in a pixel has a value lower than the said threshold value andfor sending a blemish signal to the look-up table on detection of alower value sample.
 10. Apparatus according to any of claims 1 or 2,wherein the correction factor deviation means comprises sampling meansfor sampling each pixel of the correction map of plurality of times thesamples being taken from a number of different locations within thepixel and averaging means for averaging the sum of all the samples. 11.Apparatus according to claim 10, further comprising means for generatinga low resolution test signal, and means for storing data comprising thetest signal corresponding to any given pixel of the correction pixel mapat the same memory location as the correction factor and blemish signalfor that pixel.
 12. Apparatus according to claim 10, further comprisingmeans for generating a high resolution test signal, and means forstoring data comprising the test signal at the memory location of and inplace of correction factors and blemish signals of the correction pixelmap to which particular pixels of the test signal correspond. 13.Apparatus according to claim 10, further comprising color effectgenerating means including means for loading into the correction factordetermining means of each color component, predetermined color data, andmeans for mixing the predetermined data for each component to obtain adesired color tinting of the displayed image.
 14. Apparatus according toany of claims 1 or 2, wherein the storage means comprises n dynamicrandom access memory units, where n is at least three, each arranged tobe loaded with identical data and each having an access cycle comprisingan address period, an access period and a data output period, means forapplying sequentially a different phase of an n phase clock memory unit,whereby during the access cycle of any memory unit data is read from aplurality of memory units, characterized by means for refreshing each ofthe memory units including means arranged to read the contents of atleast three memory units and to write back into each memory location ofall n memory units the most common data stored at that memory locationin the read memory units.
 15. Apparatus according to claim 14,comprising a latch associated with each memory unit, means for enablingeach latch sequentially to accept data from its associated memory unitand means for switching each latch to put data onto a common output busduring the data cycle of the associated memory unit.
 16. Apparatusaccording to claim 14, wherein n is 4 and the data cycle of memory issequentially equal to a quarter of the access cycle of each memory unit.17. Apparatus according to claim 14, wherein n is 4 and the data cycleof memory is sequentially equal to a quarter of the access cycle of eachmemory unit.
 18. A method of weighting a video signal to compensate fordefects and losses in a cathode ray tube (CRT) flying spot scanningapparatus having a scanning area for being scanned with the flying spotfor producing the signal, the method comprising the steps of:dividingthe scanning area scanned to produce a video signal by the flying spotof the CRT into an image scanning map and a digital correction maphaving a plurality of uniquely addressable pixels, the correction mapbeing derived from a digital scanning map of the CRT and beingaddressable by the scan coordinates of the scanning map, the orientationof the scanning map controlling the image scanning performed by thescanning apparatus, and the orientation of the scanning map and thecorrection map with respect to one another being variable; deriving foreach pixel of the correction map a correction factor indicative oflosses and defects associated with that pixel, the correction factorapplied to a given image scanning map pixel being derived from thecorrection map pixel which encloses the centre of that image scanningmap pixel; and storing in a store the derived correction factors andapplying the correction factors to said video signal during operation ofthe scanning apparatus to produce a compensated signal.
 19. A methodaccording to claim 18, further comprising storing brightness factorscorresponding to each pixel of the correction map and modifying theinput video signal by multiplying for each pixel video data from an areaof the scanning map corresponding to the pixel with the storedbrightness factor for that pixel.
 20. A method according to claim 19,wherein the brightness factors are predetermined and loaded into a storein addition to or in place of the correction factors.
 21. A methodaccording to claim 20, wherein the brightness factors are determined tocompensate for variation in film intensity and are multiplied with thecorrection factors to provide a set of compensation factors, and thecompensation factors are applied to the video data.
 22. A methodaccording to any of claims 18 to 21, further comprising storing pictureinformation relating to a first image together with the derivedcorrection factor for each pixel and modifying a second image bymultiplying for each pixel of the image video data from an area of thescanning map corresponding to the pixel with the stored product of thefirst image picture information and the correction factor for thatpixel.
 23. A method according to any of claims 18 to 21, furthercomprising comparing the stored correction factors for pairs of adjacentpixels, marking as blemished any pair of adjacent pixels the comparisonof the correction factors of which falls outside a predeterminedthreshold value, applying the correction factors to a video signalduring operation of the scanning apparatus for pixels not marked asblemished to produce the compensated signal, suppressing the correctionfactors for the pixels marked as blemished and substituting for thosepixels, video data derived from adjacent pixels.
 24. A method accordingto claim 23, further comprising reading an area of the correction factorstore, comparing adjacent pixels in that area, storing the location ofany pixels marked as blemished and adding to the store locations ofblemished pixels a blemish flag.
 25. A method according to claim 23,wherein the correction factors and blemish signals are loaded into astore and the video signal is corrected by multiplying for each pixelvideo data from an area of the scanning map corresponding to the pixelfor any pixel not flagged or marked as blemished with the storedcorrection factor for the pixel and by substituting corrected video datafrom the previous pixel for any pixel flagged or marked as blemished.26. A method according to claim 25, wherein a low resolution testpattern is loaded into the store in addition to the correction factorsand blemish signals.
 27. A method according to claim 25, wherein thecorrection factors and blemish signals are substituted by a highresolution test pattern.
 28. A method according to any of claims 18 to21, wherein the correction factors are derived by applying a uniformelectron beam to the unobstructed scanning area and measuring the outputfrom each pixel, the correction factor being proportional to the loss ofintensity of the measured output from a maximum output, and wherein ifthe result of the comparison of the output for a given pixel is lessthan a predetermined threshold proportion of the maximum value, thatpixel is flagged as blemished and the correction factor derived for thatpixel is suppressed.
 29. A method according to claim 28, wherein thevideo signal for an area of the scan corresponding to a pixel of thecorrection map flagged as blemished is compensated by substitution ofthe compensated signal for an adjacent unblemished pixel.
 30. A methodaccording to any of claims 18 to 21, wherein a separate set ofcorrection factors is derived for and applied to each component of thevideo signal and wherein predetermined data is loaded into thecorrection factor memory for each color component of the video signaland the contents of the memory are applied to the video data to producedesired color effects.
 31. A method according to any of claims 18 to 21,wherein the correction factor for each pixel is derived by sampling eachpixel a plurality of times and averaging the sum of all the samples,each sample being taken from a different location within the pixel. 32.A method according to claim 31, wherein each sample is compared with asecond predetermined threshold value, and, if any sample falls belowthis threshold value the pixel is flagged or marked as blemished and thecorrection factor derived for that pixel is suppressed.
 33. A method ofcorrecting a video signal produced by a telecine to compensate forvariations in response in the film from which the video signal is to bederived, comprising deriving a set of correction factors by the methodof any of claims 18 to 21, deriving a set of brightness factorscorresponding to the characteristics of the film, generating a set ofcompensation factors by combining the brightness and correction factorsand applying the compensation factors to the video data.
 34. A methodaccording to claim 33, wherein the brightness factors correspond to apre-selected response curve across the width of the film.
 35. A methodaccording to claim 33, wherein the brightness factors are derived fromthe clear film inserted in the film gate after derivation of thecorrection factors in the open gate condition.
 36. A method according toclaim 35, wherein the address for each memory is provided by a commonaddress bus.
 37. A method according to any of claims 18 to 21, whereindata is read from the store by loading n dynamic random access memoryunits with identical data where n is at least three, each random accessunit having an access cycle comprising an address period, an accessperiod and a data output period, addressing each memory unitsequentially on a different phase of an n-phase address clock, andreading data sequentially from each memory unit, whereby during theaccess cycle of any one memory unit, data is read from a plurality ofmemory units, characterized by refreshing the memory units by readingdata stored at each memory location of at least three memory units andwriting into the corresponding memory location of each memory unit themost commonly occurring data at each of the read memory locations.
 38. Amethod according to claim 37, wherein data output from each memory unitis fed into a respective latch for that memory unit, and each latch isenabled in sequence to accept data from its associated memory unit andis switched to put data onto a common output bus during the data cycleof the memory unit.